1. Field of the Invention
The present invention relates to a semiconductor film having a polycrystal structure in which crystals mass with diverse orientations as in a polycrystalline semiconductor film. The invention also relates to a semiconductor device whose active region is formed of this semiconductor film and to a method of manufacturing the semiconductor device. In particular, the present invention is suitable for a method of manufacturing a thin film transistor formed a channel formation region in this semiconductor film. The term semiconductor device herein refers to a semiconductor device in general which utilizes semiconductor characteristics to function, and semiconductor integrated circuits, electro-optical devices, and electronic equipment mounted with the semiconductor integrated circuits or the electro-optical devices fall within this category.
2. Description of the Related Art
A technique has been developed for manufacturing a thin film transistor (hereinafter referred to as TFT) from a semiconductor film that has a polycrystal structure (the film is hereinafter referred to as crystalline semiconductor film) and is formed on a glass, quartz or other substrate. A TFT formed from a crystalline semiconductor film is applied to flat panel displays, typically, liquid crystal display devices, as measures for realizing high definition image display, and is applied to monolithic displays in which a pixel portion and an integrated circuit necessary to drive the pixel portion are formed on the same substrate, as measures for realizing it.
A known alternative to SOI (silicon on insulator technology) in forming a crystalline semiconductor film is to use vapor growth method (CVD) in which a crystalline semiconductor film is formed by direct deposition on a substrate, or to crystallize an amorphous semiconductor film by heat treatment or laser light irradiation. If the formed crystalline semiconductor film is to be applied to a TFT, the latter method is employed more often because the method provides the TFT with excellent electric characteristics.
A crystalline semiconductor film can have a polycrystal structure if it is obtained by subjecting an amorphous semiconductor film formed on a glass, quartz or other substrate to heat treatment or laser light irradiation for crystallization. Crystallization is known to progress from a crystal nuclear spontaneously generated in the interface between the amorphous semiconductor film and the substrate. While crystal grains in a polycrystal structure each educe an arbitrary crystal plane, it has been found that the proportion, which the crystallization of the {111} plane requiring the minimum interface energy is educed, is high if silicon oxide is placed under the crystalline semiconductor film.
The thickness of a semiconductor film required for TFT is about 10 to 100 nm. However, it is difficult in this thickness range to control crystal orientation in the interface between the semiconductor film and a substrate that is formed from a different material due to lattice discordance or crystal nuclei generated irregularly. Also, it has been impossible to increase the grain size of each crystal grain because of mutual interference between crystal grains.
Another method of forming a crystalline silicon film has been disclosed in which an element for promoting crystallization of silicon is introduced into an amorphous silicon film, thereby obtaining a crystalline silicon film through heat treatment at a temperature lower than in prior art. For example, Japanese Patent Application Laid-open Nos. Hei 7-130652 and Hei 8-78329 describe obtaining a crystalline silicon film by introducing nickel or other metal element into an amorphous silicon film and subjecting the film to heat treatment at 550xc2x0 C. for four hours.
In this case, the element introduced at a temperature lower than the temperature at which a natural nuclear is generated forms silicide, and crystal growth starts from this silicide. For instance, when the element is nickel, nickel silicide (NiSix (0.4 less than x less than 2.5) is formed. While nickel silicide has no specific orientation, it advances crystal growth in an amorphous silicon film almost only in the direction parallel to the substrate if the thickness of the film is 10 to 100 nm. In this case, the interface energy of the interface between NiSix and the {111} plane of the crystalline silicon is the smallest, and hence the plane parallel to the surface of the crystalline silicon film is the {110} plane to orient crystals mainly in the {110} plane orientation. However, when the crystal growth direction is parallel to the substrate surface and a crystal grows into a pillar, the crystal may not always be oriented in the {110} plane orientation because there is a degree of freedom in the rotation direction as axis of the pillar-like crystal. Accordingly, other lattice planes are deposited.
When the orientation ratio is low, continuity of lattices cannot be maintained in a crystal grain boundary where crystals of different orientations meet one another, resulting in formation of many dangling bonds. The dangling bonds formed in the crystal grain boundary acts as recombination center or trap center, to thereby lower the carrier (electrons or holes) transportation characteristic. As a result, carriers are lost in recombination or trapped by defects. If a crystalline semiconductor film as such is used to form a TFT, the TFT cannot have high electric field effect mobility.
Also, controlling positions of crystal grains as desired is nearly impossible and crystal grain boundaries are placed irregularly, which does not allow a TFT to form its channel formation region solely from crystal grains of a specific crystal orientation. This lowers the continuity of crystal lattices and forms defects in crystal grain boundaries, thereby causing fluctuations in TFT characteristics and presenting various adverse influences. For instance, the field effect mobility is degraded to make the TFT incapable of operating at high speed. In addition, a fluctuation in threshold voltage is an obstruction to low voltage driving, leading to an increase in power consumption.
The present invention has been made to present solutions to those problems, and an object of the present invention is therefore to raise the orientation ratio of a crystalline semiconductor film obtained by crystallizing an amorphous semiconductor film through heat treatment and irradiation of intense light such as laser light, ultraviolet rays, or infrared rays and to provide a semiconductor device whose active region is formed from the crystalline semiconductor film and a method of manufacturing the semiconductor device.
In order to solve the above problems, the present invention uses a semiconductor film containing silicon and germanium as its ingredient and having a crystal structure, the semiconductor film having the {101} plane that reaches 30% or more of all the lattice planes detected by reflection electron diffraction pattern method. This semiconductor film is obtained by forming an amorphous semiconductor film containing silicon and germanium as its ingredient through plasma CVD in which hydride, fluoride, or chloride gas of a silicon element is used, the repetition frequency is set to 10 kHz or less, and the duty ratio is set to 50% or less for intermittent electric discharge or pulsed electric discharge, and by introducing an element for promoting crystallization of this amorphous semiconductor film to the surface thereof to crystallize the amorphous semiconductor film through heat treatment, or through heat treatment and irradiation of intense light such as laser light, ultraviolet rays, or infrared rays, while utilizing the introduced element. The semiconductor film having a crystal structure can be used for an active layer such as a channel formation region.
The thus formed semiconductor film containing silicon and germanium and having a crystal structure contains Group 14 (new international notation) elements in the periodic table other than silicon in a concentration of 1xc3x971018 atoms/cm3 or below. The semiconductor film contains less than 5xc3x971018 nitrogen atoms per cm3, less than 5xc3x971018 carbon atoms per cm3, and less than 1xc3x971019 oxygen atoms per cm3.
The element for promoting crystallization is one or more elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au. The thickness of the amorphous semiconductor film is set to 10 to 100 nm. An amorphous silicon film is doped with the metal element given in the above and subjected to heat treatment to form a compound of silicon and the metal element (silicide). Formation of the compound spreads to advance crystallization. Germanium contained in the amorphous silicon film does not react with this compound and generates local distortion by being present around the compound. This distortion acts to increase the critical radius of nuclear generation and to reduce the nuclear generation density. The distortion also has an effect of limiting orientation of crystals.
The concentration of germanium needed to exhibit those effects has been found to be 0.1 atomic percent or more and 10 atomic percent or less, preferably 1 atomic percent or more and 5 atomic percent or less, with respect to silicon, as a result of experiments. If the germanium concentration exceeds this upper limit, silicon and germanium reacts and form an alloy to generate a large number of natural nuclei (spontaneously generated nuclei other than the compound of silicon and the metal element used for the doping), and a polycrystalline semiconductor film obtained cannot have a high orientation ratio. On the other hand, if the germanium concentration is lower than the lower limit, the distortion generated is not enough to raise the orientation ratio.
The amorphous silicon film doped with germanium is formed by plasma CVD using intermittent electric discharge or pulsed electric discharge. The intermittent electric discharge or pulsed electric discharge is obtained by modulating high frequency power with an oscillation frequency of 1 to 120 MHz, preferably 13.56 to 60 MHz, into power with a repetition frequency of 10 Hz to 10 kHz and by supplying the modulated power to a cathode. When the duty ratio is defined as the ratio of time during which high frequency power application lasts to one cycle of the repetition frequency, the duty ratio is set to 1 to 50%.
The intermittent electric discharge or pulsed electric discharge as above allows selection of radical species (meaning here atoms or molecules that are electrically neutral and chemically active) in the deposition process of the amorphous semiconductor film, so that a film can be formed from a radical species having a relatively long life period. For example, various radical species and ion species are generated when dissolving SiH4 in an electric discharge space. Radical species repeat generation and extinguishment reactions but electric discharge that is continued steadily keeps the existence proportions of radical species fixed. On the other hand, if there is a period where electric discharge is stopped as in intermittent electric discharge or pulsed electric discharge, only radical species that has longer life period is supplied due to the difference in life period between the radical species and ion species to the film deposition surface and is used to form the film.
A long-living radical is chosen in order to inactivate the film growth surface and is suitable for dispersing and including germanium throughout the amorphous silicon film. GeH4, which is a germanium source, is smaller in dissolution energy than SiH4, and hence generates atomic-state germanium when dissolved with the same supply power as SiH4, forms germanium clusters through vapor reaction or surface reaction. Dispersed germanium is preferred according to the crystal growth model described above, which leads to the conclusion that intermittent electric discharge in which no cluster is generated is preferable.
The amorphous semiconductor film loses its volume when crystallized due to rearrangement of atoms. As a result, the polycrystalline semiconductor film on the substrate contains tensile stress. However, the volume shrinkage accompanied crystallization can be limited and the internal stress generated can be reduced by making the amorphous semiconductor film contain germanium with a large atom radius in 0.1 atomic percent or more and 10 atomic percent or less, preferably 1 atomic percent or more and 5 atomic percent or less, with respect to silicon. At this point, germanium is contained preferably in a dispersed state in order to obtain a uniform effect throughout the film.